While indium tin oxide, indium oxide, tin oxide, zinc oxide, etc. are all used as transparent electrodes of LCDs and electroluminescent display elements, indium tin oxide (called ITO hereinafter) is mainly used for this purpose. Examples of conventional etching liquids for ITO-films are 1. aqueous solutions of iron (III) chloride, 2. aqueous solutions of iodic acid, 3. aqueous solutions of phosphoric acid, 4. mixed solutions of hydrochloric acid and nitric acid (aqua regia), 5. aqueous solutions of oxalic acid, etc. However, these etching liquids are not adequate for practical use because of the following problems:
1. Aqueous solutions of iron (III) chloride are inexpensive and have a high etching speed; however, they have the disadvantage that with these solutions the amount of side-etching is high and that they contain Fe which has a negative influence on semiconductors.
2. With aqueous solutions of iodic acid, which have good etching characteristics, the amount of side-etching is low; however, in addition to being expensive, they lack stability because iodine is easily released.
3. Aqueous solutions of phosphoric acid, in addition to etching A1 used on wiring, have the disadvantage that a residue remains after etching.
4. Mixed solutions of hydrochloric acid and nitric acid (aqua regia) change rapidly over time, making process control difficult and delivery impossible.
5. Aqueous solutions of oxalic acid have many advantages such as excellent stability, low cost, and the fact that they do not etch A1, etc.; however, a residue remains after etching with these solutions.
Regarding the residues which occur when using an aqueous solution of oxalic acid, JP, A, 7-141932 discloses a technique for reducing the residues by using dodecylbenzene-sulfonic acid; however, this has the disadvantage of intense foaming.
Moreover, the process of forming a silicon nitride film etc. on a glass substrate and forming ITO film thereon is examined in these years, and the structure of the element is changing. Therefore, removing property of the residue is becoming insufficient.